The present invention relates to a method, provided in particular for road vehicles, for controlling the braking pressure variation in the wheel cylinders or vehicular brakes with brake slip control. The wheel rotational behaviour and the vehicle velocity, or a reference value representative of the vehicle velocity, are determined and logically combined to generate control signals by which the braking pressure at the wheels is controlled, individually and/or by groups. As soon as a tendency to lock is recognized, the braking pressure is decreased, maintained constant, or increased by the actuation of valves inserted in the pressure fluid circuit, in response to deceleration and/or slip thresholds. Devices for implementing this method are likewise the subject of the present invention.
As is commonly known, brake slip control apparatuses prevent the vehicle wheels from locking even in the event of heavy braking, in particular in the event of emergency or panic stops. Instead, the wheels continue to roll in a favourable slip range, with a view to preserving vehicle stability even under extreme conditions and rendering the stopping distance as short as possible. Brake slip control can be individually adjusted for each wheel, but for reasons of low-cost manufacture may be confined to sensing the wheel rotation and/or the pressure control of just some wheels. Or only certain parallel wheel groups, e.g. the wheels of the rear axle, may be slip controlled. It has been proposed to build up by delay, or to prematurely decrease, the wheel cylinder pressure at wheels not sensed, or not controlled, in dependence upon the pressure of the controlled wheels (German patent specification 26 27 284).
Many "control philosophies" and circuits are known for controlling braking pressure variation during a braking action based on the wheel's rotational behaviour, and for determining and processing the necessary measured values and for generating the control signals. But in methods and devices with continuous pressure modulation, or with invariable pressure control pulses, the system-inherent delay periods often have detrimental effects. Under bad conditions, e.g. on icy roads or wet surface aquaplaning, a pressure decrease initiated too late will not prevent quick locking of the wheel. In other cases, an excessive reduction of the braking pressure has to be immediately corrected by a rapid re-increase. However, marked variations in braking pressure during controlled braking impair driving stability, increase stopping distance, and reduce driving comfort.
German patent specification 19 14 765 described an electric control for an anti-locking brake system for a vehicle. When the wheel deceleration exceeds a predetermined magnitude, a pulse generator circuit feeds actuating signals to a solenoid valve assembly composed of inlet and outlet valves. The pulse generator circuit serves to stepwise decrease or build up the braking pressure. The ratio of (pulse length)/(pulse spacing) of the actuating signals is adapted to vary between 0 and infinity in response to the wheels' rotional behaviour. But such control behaviour critically depends upon how quickly the measured values for the control can be determined, processed, and corrected. It also depends on how fast they may take effect on the actual braking pressure distribution after having overcome the system-inherent dead times.
It is an object of the present invention to overcome these shortcomings by a braking method with slip control wherein the modulation of pressure fluid is comparatively small, reducing pressure variations in the individual wheel cylinders.